In a so-called air-bridge type piezoelectric thin film resonator, in order to acoustically separate a vibration portion, which is formed of a pair of excitation electrodes facing each other and a piezoelectric thin film provided therebetween, from a substrate, the structure is formed in which a thin film member (membrane) partly floats over the substrate with an airspace layer provided there between.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 61-218214
Heretofore, planarization has not been studied for an air-bridge type piezoelectric thin film resonator.
The orientation property of a piezoelectric thin film influences resonant properties. In order to improve the resonant properties, the orientation property of a piezoelectric thin film must be improved, and in order to improve the orientation property thereof, the flatness of a structural film, which is provided under an excitation electrode, must be improved.
As a method for planarizing the structural film provided under the excitation electrode, for example, polishing by CMP (chemical mechanical polishing) may be mentioned.
In an air-bridge type piezoelectric thin film resonator, a structural film is formed on a sacrifice layer which is used for forming an airspace layer. Hence, irregularities are formed on the structural film which is to be polished. By polishing, convex portions (projecting surfaces) can only be planarized, and in addition, since slurry is trapped in concave portions, peripheries of the convex portions are likely to be polished as compared to central portions thereof. Accordingly, even when the structural film on the sacrifice layer is polished, film-thickness distribution is generated, and as a result, superior resonant properties cannot be obtained. In order to avoid this problem, a method may be used in which after a burying material is buried in concave portions, planarization is performed by CMP, and the burying material is then removed; however, in this case, the manufacturing process becomes complicated, and hence the manufacturing cost is increased.
In addition, when a plurality of piezoelectric thin film resonators is simultaneously formed using a wafer, it is difficult to uniformly polish structural films of individual resonators in the wafer on the order of nanometers in thickness. The resonant frequency and the resonant properties of a resonator largely depend on the thickness of the structural film. When the thicknesses of the structural films vary within a wafer, the yield may be decreased, or the number of steps, such as frequency adjustment, may be increased, and as a result, the manufacturing cost is increased.